The invention relates generally to semiconductor lasers (laser diodes), and more specifically to techniques for stabilizing the wavelength of such lasers in connection with use in fiber optic communications systems.
It is well established that multiple optical communications channels can be optically multiplexed onto a single optical fiber by a technique known as wavelength division multiplexing (WDM). Multiple lasers, each at a different wavelength from the others, are modulated in accordance with respective information patterns, and the modulated light from each laser is output on a respective fiber segment. The light from all the fiber segments is combined onto a single fiber by a wavelength division multiplexer. Light at the other end of the single fiber is separated onto individual fiber segments by a wavelength division demultiplexer, and the light on the individual fiber segments is demodulated to recover the original information patterns.
As the demand for bandwidth has, exploded, due in large part to the growth of the Internet and data communications, additional demands are made on the fiber optic technology. A relatively new technology, called dense wavelength division multiplexing (DWDM), is being deployed to expand the capacity of new and existing optical fiber systems. The improvements include providing more wavelength channels, and where possible, increasing the bit rate on each channel.
As is well known, typical single-mode fiber optics communications are at wavelengths in the 1300-nm and 1550-nm ranges. The International Telecommunications Union (ITU) has defined a standard wavelength grid having a frequency band centered at 193,100 GHz, and other bands spaced at 100 GHz intervals around 193,100 GHz. This corresponds to a wavelength spacing of approximately 0.8 nm around a center wavelength of approximately 1550 nm, it being understood that the grid is uniform in frequency and only approximately uniform in wavelength. Implementations at other grid spacings (e.g., 25 GHz, 50 GHz, 200 GHz, etc.) are also permitted.
A given fiber in a communications system may need to carry as many as 80 closely spaced wavelengths. As the bit rates increase and the wavelength channels become more closely spaced, crosstalk becomes an increasing problem. Thus the need to control the lasers' output wavelengths has become more critical. For example, in a system with the wavelengths spaced at 50 GHz (0.4 nm), each laser's wavelength should be stabilized to a small fraction of the wavelength spacings on the grid. In the example of wavelengths spaced at 50 GHz, this means that an individual laser should be stabilized to within about 2.5 GHz (0.02 nm).
One type of laser source for fiber optic communications systems is what is known as an external cavity laser diode (ECLD). An ECLD includes a laser diode chip in combination with an external waveguide formed with a grating. The grating acts as a filter and limits the output wavelengths to a band that is much narrower than the laser diode's inherent range of wavelengths. A particular type of ECLD uses a fiber Bragg Grating (FBG). It is known that the output wavelength of an ECLD depends on the optical pitch of the grating, which depends on the geometric pitch of the grating and the refractive index of the fiber in the grating region. The geometric pitch and refractive index vary with temperature in accordance with the thermal and material characteristics of the fiber.
Given that laser source modules are typically specified for wide ambient temperature ranges (e.g., −10° C. to +70° C., or even −40° C. to +85° C.), it is known practice to provide a temperature sensing and temperature controlling devices in the module package. A widely used cooling device is a thermoelectric cooler (TEC).